X.121 ITU-T standard describing an addressing scheme used in X.25 networks. X.121 addresses are sometimes called IDNs (International Data Numbers). X.21 ITU-T standard for serial communications over synchronous digital lines. The X.21 protocol is used primarily in Europe and Japan. X.21bis ITU-T standard that defines the physical layer protocol for communication between DCE and DTE in an X.25 network. Virtually equivalent to EIA/TIA-232. See also EIA/TIA-232 and X.25. X.25 ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs. X.25 specifies LAPB, a data link layer protocol, and PLP, a network layer protocol. Frame Relay has to some degree superseded X.25. See also Frame Relay, LAPB, and PLP. X.25 Level 3 See PLP. X.25 Protocol See PLP. X.28 ITU-T recommendation that defines the terminal-to-PAD interface in X.25 networks. See also PAD and X.25. X.29 ITU-T recommendation that defines the form for control information in the terminal-to-PAD interface used in X.25 networks. See also PAD and X.25. X.3 ITU-T recommendation that defines various PAD parameters used in X.25 networks. See also PAD and X.25. X3T9.5 Number assigned to the ANSI Task Group of Accredited Standards Committee for their internal, working document describing FDDI. X.400 ITU-T recommendation specifying a standard for electronic mail transfer. X.500 ITU-T recommendation specifying a standard for distributed maintenance of files and directories. X.75 ITU-T specification that defines the signalling system between two PDNs. X.75 is essentially an NNI. See also NNI. X Display Manager Control Protocol See XDMCP. Xerox Network Systems See XNS. XID Exchange identification. Request and response packets exchanged prior to a session between a router and a Token Ring host. If the parameters of the serial device contained in the XID packet do not match the configuration of the host, the session is dropped. Xid See termid. XDMCP X Display Manager Control Protocol. Protocol used to communicate between X terminals and workstations running UNIX. XNS Xerox Network Systems. Protocol suite originally designed by PARC. Many PC networking companies, such as 3Com, Banyan, Novell, and UB Networks used or currently use a variation of XNS as their primary transport protocol. XRemote Protocol developed specifically to optimize support for X Windows over a serial communications link. XStream Major public PSN in the United States operated by MCI. Formerly called TYMNET. X terminal Terminal that allows a user simultaneous access to several different applications and resources in a multivendor environment through implementation of X Windows. See also X Windows. X Windows Distributed, network-transparent, device-independent, multitasking windowing and graphics system originally developed by MIT for communication between X terminals and UNIX workstations. See also X terminal.